Field of the Invention
This invention relates to an improvement in a solid state image sensing device. More particularly, this invention relates to an improvement in a solid state sensing device comprising on a single monolithic substrate a large number of light sensing parts and scanning circuit part for time-sequentially reading out the output signals of the light sensing parts.
The most important characteristic required for a solid state image sensing device is high resolving power, and for the achievement of which MOS-LSI technology becomes widely used since it enables high integration of the device as well as monolithic construction of the light sensing part and scanning circuit part.
FIG. 1 shows one example of the conventional solid stage image sensing device, wherein numeral 1 designates a horizontal scanning circuit which generally is a MOS shift register driven by clock pulses, and numeral 2 designates a vertical scanning circuit which also generally is a MOS shift register driven by clock pulses. These scanning circuits 1 and 2 are generally driven by a 2-phase clock pulses, and the start pulse given to the first stages of the shift registers in the scanning circuits are shifted one by one at specified timings to subsequent stages, thereby sequentially issuing scanning pulse signals to their output lines 3, 3, . . . and 28, 28, . . . , respectively. The horizontal scanning circuit 1 and the vertical scanning circuit 2 thus drive the horizontal MOS switches 23, 23 . . . and the vertical MOS switches 21, 21 . . . in the order of from left to right and from upper to lower, by their output scanning signals, respectively. The output signal of the photo-diodes 27, 27, . . . are taken out through the vertical MOS switches 21, 21, . . . at the lines 22, 22, . . . And the signals at the lines 22, 22 . . . are taken out at the video signal output terminal 35 in turn by the horizontal MOS switches 23, 23 . . . The video signal sent out to the terminal 35 consists of train of pulses, and each of the pulse is produced by a charging current from the power source 36 to the photo-diodes 27. The charging current charges the photo-disode 27 that has been discharging as a result of receiving light for a time period for one frame scanning. The signal thus made by the photo-diode 27 corresponds to the light incident thereon. The video signal, i.e., the charging current is read out as voltage signal across the load resistor 37.
The abovementioned known solid state image sensing device is simple in construction, and therefore can be formed on a semiconductor monolithic substrate by utilizing LSI technology, but has some problems at actual operation.
A first one of such problem of the conventional solid state image sensing device of FIG. 1 is that the video signal produced by the device likely to contain spurious signal or light spurious signal. The reason of containing such spurious signal is as follows: All of the drain electrodes of the vertical MOS switches 21, 21 . . . are connected to respective corresponding vertical lines i.e., connecting lines 22, 22, . . . As already known, in the solid state image sensing device the photo-diodes 27, 27 . . . are generally formed by the source regions of the vertical MOS switches 21, 21 . . . Namely, the source regions per se, which are diffused region of opposite conductivity type to the substrate, region with a p-n junctions inbetween, form the photo-diodes. Therefore, the drain regions have substantially the same construction with the source regions forming the photo-diodes. Accordingly, in case the solid state image sensing device is made compact with high integration and hence the distance between the neighboring sensing elements become extremely short, even though the elements are optically isolated from each other, many carriers are generated by an intense light in the part of the substrate immediately under a source region (photo-diode region) which is shot by the intense light, and part of the carriers reach nearby drain regions around the source region. Thus, the drain regions which are near the intensely illuminated source but do not receive light themselves issue spurious currents.
A second problem of the conventional solid state image sensing device of FIG. 1 is fixed pattern noise. This noise is caused by that, when drain of some of the vertical MOS switch 21 of FIG. 1 has a large dark current failure, signals of all the photo-diodes which are connected to the same vertical line with the defect MOS switch are badly affected, thereby producing white vertical line noise in the monitor picture.
A third problem of the conventional device is that it has a considerable large stray capacitance on the lines 22, 22 . . . , since a large number of drains connected thereto give a considerable large junction capacitance. Therefore, in such conventional devices, it has been difficult to achieve a satisfactory high speed in operating, for example, a large device having 254.times.244 sensing elements.